Comparison of different uncertainty techniques in urban stormwater quantity and quality modelling

Urban drainage models are important tools used by both practitioners and scientists in the field of stormwater management. These models are often conceptual and usually require calibration using local datasets. The quantification of the uncertainty associated with the models is a must, although it is rarely practiced. The International Working Group on Data and Models, which works under the IWA/IAHR Joint Committee on Urban Drainage, has been working on the development of a framework for defining and assessing uncertainties in the field of urban drainage modelling. A part of that work is the assessment and comparison of different techniques generally used in the uncertainty assessment of the parameters of water models. This paper compares a number of these techniques: the Generalized Likelihood Uncertainty Estimation (GLUE), the Shuffled Complex Evolution Metropolis algorithm (SCEM-UA), an approach based on a multi-objective auto-calibration (a multialgorithm, genetically adaptive multi-objective method, AMALGAM) and a Bayesian approach based on a simplified Markov Chain Monte Carlo method (implemented in the software MICA). To allow a meaningful comparison among the different uncertainty techniques, common criteria have been set for the likelihood formulation, defining the number of simulations, and the measure of uncertainty bounds. Moreover, all the uncertainty techniques were implemented for the same case study, in which the same stormwater quantity and quality model was used alongside the same dataset. The comparison results for a well-posed rainfall/runoff model showed that the four methods provide similar probability distributions of model parameters, and model prediction intervals. For ill-posed water quality model the differences between the results were much wider; and the paper provides the specific advantages and disadvantages of each method. In relation to computational efficiency (i.e. number of iterations required to generate the probability distribution of parameters), it was found that SCEM-UA and AMALGAM produce results quicker than GLUE in terms of required number of simulations. However, GLUE requires the lowest modelling skills and is easy to implement. All non-Bayesian methods have problems with the way they accept behavioural parameter sets, e.g. GLUE, SCEM-UA and AMALGAM have subjective acceptance thresholds, while MICA has usually problem with its hypothesis on normality of residuals. It is concluded that modellers should select the method which is most suitable for the system they are modelling (e.g. complexity of the model’s structure including the number of parameters), their skill/knowledge level, the available information, and the purpose of their study.     

In vitro fermentation studies for selection and evaluation of Bacillus strains as starter cultures for the production of okpehe, a traditional African fermented condiment

Selected Bacillus and Enterococcus strains, isolated from traditional okpehe fermentations, were studied for their suitability as starter cultures in laboratory-scale fermentations of Prosopis africana seeds for the production of okpehe, a traditional fermented vegetable product of Nigeria. The strains were selected on the basis of highest proteolytic activity, as determined with the APIZYM (BioMerieux) test. The choice of starter strains was narrowed to Bacillus subtilis strains BFE 5301 and BFE 5372. These were determined as the best starter combination because of rapid growth, high amylolytic and proteolytic activities, high levels of polyglutamic acid production by strain BFE 5372, as well as bacteriocin production by strain BFE 5301. Other mixed culture fermentations did not yield sensorically acceptable products. Although a monoculture fermentation, using only B. subtilis strain BFE 5372, produced okpehe with very good sensory characteristics, the growth of B. cereus could be detected after 48 h fermentation, indicating that this starter did not sufficiently contribute to product safety. Mixed culture fermentation with the combination of bacteriocin-producing starter B. subtilis BFE 5301 and the non-bacteriocin-producing B. subtilis BFE 5372, produced a product with good sensory characteristics, in which growth of B. cereus was delayed. The bacteriocin produced by B. subtilis strain BFE 5301 was identified as subtilisin, using subtilisin-specific primers and PCR amplification of the subtilisin gene. The bacteriocin was heat-stable at 100 degrees C for 10 min and exhibited highest activity at pH values lower or equal to pH 6.0. The bacteriocin was sensitive to the proteolytic enzymes trypsin and alpha-chymotrypsin at concentrations of 10 mg/ml.     

SiGe epitaxy on a 300 mm batch furnace

This work reports the feasibility of silicon and silicon germanium epitaxy using an ASM A412(TMa) LPCVD all quartz, hot wall, vertical batch furnace reactor using 100 wafer product loads. The very same furnace can be used for 25 wafer and 200 wafer load size, without any hardware changes, dependant on production needs. Following this approach a significant cost reduction for epitaxy in 300 mm high volume manufacturing is possible and enables new applications. The native oxide of the substrate was removed by wet chemical cleaning with time coupling of less than 1 h and subsequent in-situ low pressure hydrogen anneal prior to Si or SiGe deposition. The epitaxial layers were grown using silane and germane. The Si and SiGe layers have been characterized with ToFSIMS, XRD, Raman, AFM and TEM confirming excellent crystalline quality, layer thickness and within wafer SiGe stoichiometry uniformity.     

In vitro adherence and other properties of lactobacilli used in probiotic yoghurt-like products

This study compared selected probiotic properties of 9 strains of the Lactobacillus acidophilus group (6 L. acidophilus and 3 L. johnsonii) and 9 strains of the L. casei group, isolated from probiotic dairy products sold on the German market or supplied by a producer of starter cultures. Using an in vitro system simulating gastric transit, strains of the L. acidophilus group were more tolerant to the low pH 2.0 than strains of L. paracasei and L. rhamnosus which rapidly lost their viability after exposure to the simulated gastric juice containing pepsin. Milk showed a protective effect on strains less resistant to the gastric acidity. Bile salt hydrolase activity was detected in all strains of L. acidophilus/L. johnsonii but not in strains of the L. casei group. All lactobacilli were able to adhere to mucus secreting HT29 MTX cells and to human collagen type IV, fibrinogen and fibronectin although differences in the percentage of adhering bacteria were observed between different strains.     

Enterococci in foods--a conundrum for food safety

Enterococci form part of the lactic acid bacteria (LAB) of importance in foods. They can spoil processed meats but they are on the other hand important for ripening and aroma development of certain traditional cheeses and sausages, especially those produced in the Mediterranean area. Enterococci are also used as human probiotics. However, they are important nosocomial pathogens that cause bacteraemia, endocarditis and other infections. Some strains are resistant to many antibiotics, but antibiotic resistance alone cannot explain the virulence of some of these bacteria. Virulence factors such as adhesins, invasins and haemolysin have been described. The role of enterococci in disease has raised questions on their safety for use in foods or as probiotics. Studies on the incidence of virulence traits among enterococcal strains isolated from food showed that some harbour virulence traits and generally, Enterococcus faecalis harbours more of them than Enterococcus faecium. Regulations in Europe stipulate that safety of probiotic or starter strains is the responsibility of the producer; therefore, each strain intended for such use should be carefully evaluated. For numerous questions, immediate answers are not fully available. It is therefore suggested that when considering an Enterococcus strain for use as a starter or probiotic culture, it is imperative that each particular strain should be carefully evaluated for the presence of all known virulence factors. Ideally, such strains should harbour no virulence determinants and should be sensitive to clinically relevant antibiotics. In general, E. faecium appears to pose a lower risk for use in foods, because these strains generally harbour fewer recognised virulence determinants than E. faecalis. Generally, the incidence of such virulence determinants among E. faecium strains is low, as compared to E. faecalis strains, probably as a result of the presence of pheromone-responsive plasmids.     

Characterization and Technological Properties of Lactic Acid Bacteria in the Production of “Sorghurt,” a Cereal-Based Product

A total of 57 predominant LAB strains isolated from cassava and maize grains fermentation processes for fufu and ogi were identified using phenotypic and genomic fingerprinting methods such as rep-PCR and ARDRA. They were divided into facultatively heterofermentative rods (26.3%), obligately heterofermentative rods (31.6%), and tetrad forming homofermentative cocci (42.1%). Selected strains were further identified by sequencing the 16S rDNA gene. Technological studies such as acidification, hydrogen peroxide production, starch hydrolysis, enzymatic activities, degradation of oligosaccharides, and in vitro adherence properties were carried out. Lactobacillus plantarum strains demonstrated better and rapid acid production capability, followed by the Pediococcus strains, while L. fermentum strains exhibited slower acid production. Hydrogen peroxide production was observed among the LAB groups. The test strains utilized the indigestible sugars raffinose and stachyose. Only L. pentosus demonstrated high amylase activity comparable to that of L. amylovorus DSM 20531. Lactobacillus plantarum and L. fermentum strains showed high β-glucosidase activity. Six strains were selected as starter cultures. The strains were tolerant to acidic pH levels and bile salt. The yoghurt-like “sorghurt” produced using the selected starter cultures had a final pH of less than pH 4.0, and a viable count of less than 5.5 Log₁₀cfu/mL at the end of a 24 h fermentation period. The samples were generally acceptable to the taste panelists. The starter organisms demonstrated varying degree of adherence to HT29 MTX cell line. Therefore, employing functionally defined LAB strains may be one practical approach for incorporating health-promoting features into appropriate food products suitable for targeted population.